1. Field of the Invention
The present invention relates to a surface acoustic wave device, and more particularly, it relates to a surface acoustic wave device using a piezoelectric thin film for forming a piezoelectric substrate.
2. Description of the Background Art
In recent years, surface acoustic wave devices such as a filter, a resonator or an oscillator using surface acoustic waves, which may hereinafter be referred to as SAW, have been widely employed.
The surface acoustic wave device is more suitable for high frequency as compared with a bulk wave device. The fundamental frequency of the surface waves generated in the surface acoustic wave device depends on the pitch of the interdigital electrodes and the propagation velocity of the surface acoustic wave (SAW velocity). In order to further increase the frequency in such a surface acoustic wave device, therefore, it is necessary to reduce the pitch of the interdigital electrodes or to employ a substrate having a high SAW velocity.
However, the possible pitch reduction of the interdigital electrodes is limited, so a high SAW velocity substrate must be used in order to further increase the frequency beyond the limit.
There has been developed a high SAW velocity substrate which comprises a single-crystalline dielectric member of silicon, sapphire or diamond and a piezoelectric thin film of zinc oxide or aluminum nitride provided thereon. Such a piezoelectric thin film must be epitaxially grown in order to reduce propagation loss in a high-frequency region.
On the other hand, aluminum is mainly used as a metal for forming the interdigital electrodes. Aluminum is thus employed for the reasons that it is easy to photolithograph, its specific gravity is so small as to provide a small electrode mass-loading effect, it has high conductivity, and the like. An aluminum film, which is generally formed by electron beam deposition (vapor deposition), sputtering or the like for defining the interdigital electrodes, is a crystallographically random-oriented polycrystalline film.
It is impossible to grow a piezoelectric thin film epitaxially on such an aluminum film, which is a random-oriented polycrystalline film. In a conventional surface acoustic wave device 10 shown in FIG. 4 using a high acoustic velocity substrate 13, therefore, a piezoelectric thin film 12 is first formed epitaxially on a single-crystalline dielectric member 11 by a method such as sputtering or CVD to obtain the high acoustic velocity substrate 13, and then aluminum electrodes 14 are formed on the surface of the piezoelectric thin film 12. In other words, the aluminum electrodes 14 are provided on the surface of the surface acoustic wave device 10 and exposed to the atmospheric air.
However, a piezoelectric thin film formed by a method such as sputtering or CVD can easily causes projections on its surface due to adhesion of dirt or the like. Therefore, it may not be possible to form smooth aluminum electrodes on the piezoelectric thin film. When photolithography is applied to an aluminum film for forming aluminum electrodes, projections may also be formed on the photoresist film to cause difficulty in giving intimate contact of a photomask with the surface of the photoresist film, which is provided on the aluminum film, because of such projections. If the projections formed on the piezoelectric thin film cause corresponding projections on the aluminum film, further, uniform application of the photoresist material may be prevented by such projections formed on the aluminum film. Thus, the aluminum electrodes formed in this way may be short-circuited.
Further, it has been recognized that, when a signal of a high voltage level is applied to a surface acoustic wave device such as an SAW filter or an SAW resonator, aluminum electrodes are subjected to strong stress by surface acoustic waves, which causes metal migration. This migration, which is caused by stress, is called stressmigration, to be distinguished from electromigration. Such stressmigration leads to electrical shorting and characteristic deterioration such as increase of insertion loss and reduction of the quality factor (Q) of the resonator. Occurrence of such stressmigration is facilitated as the frequency is increased, to cause a significant problem particularly in a surface acoustic wave device which is employed in a high-frequency region.
Although a small amount of Cu, Ti, Ni, Mg or Pd is added to an electrode material of aluminum as a countermeasure for preventing such stressmigration, sufficient improvement has not yet been attained.